Abstract: Measured reflection data such as the bidirectional texture function (BTF) represent spatial variation under the full hemisphere of view and light directions and offer a very realistic visual appearance. Despite its high-dimensional nature, recent compression techniques allow rendering of BTFs in real time. Nevertheless, a still unsolved problem is that there is no representation suited for real-time rendering that can be used by designers to modify the BTF's appearance. For intuitive editing, a set of low-dimensional comprehensible parameters, stored as scalars, colour values or texture maps, is required. In this paper we present a novel way to represent BTF data by introducing the geometric BRDF (g-BRDF), which describes both the underlying meso- and micro-scale structure in a very compact way. Both are stored in texture maps with only a few additional scalar parameters that can all be modified at runtime and thus give the designer full control over the material's appearance in the final real-time application. The g-BRDF does not only allow intuitive editing, but also reduces the measured data into a small set of textures, yielding a very effective compression method. In contrast to common material representation combining heightfields and BRDFs, our g-BRDF is physically based and derived from direct measurement, thus representing real-world surface appearance. In addition, we propose an algorithm for fully automatic decomposition of a given measured BTF into the g-BRDF representation.